1. Field of the Invention
The present invention relates to a corona charger, and a process cartridge and an image forming apparatus using the corona charger.
2. Discussion of the Related Art
In a typical electrophotographic image forming apparatus, first, a surface of a photoreceptor is evenly charged, and the charged surface is then exposed to a light beam modulated by image information to form an electrostatic latent image thereon. A toner is supplied to the electrostatic latent image to form a toner image on the surface of the photoreceptor. The toner image is transferred onto a recording medium directly or via an intermediate transfer member, and then fixed thereon upon application of heat and pressure. Residual toner particles remaining on the surface of the photoreceptor are removed by a cleaning blade.
The photoreceptor is typically charged using a corona charger.
Corona discharge is a continuous discharge phenomenon that occurs upon local dielectric breakdown of air in an uneven electric field. A typical corona charger has a configuration in which a corona wire with a micro-diameter is stretched taut in a shield case made of aluminum, a part of which is eliminated. Corona ions are discharged from the part of the sealed case which is eliminated. As the voltage applied to the corona wire increases, a strong electric field is locally formed at the periphery of the corona wire, causing local dielectric breakdown of air and thus continuous discharge of electricity.
The type of corona discharge largely depends on the polarity of the voltage applied to the corona wire. A positive corona discharge causes an even electric discharge on the surface of the corona wire, whereas a negative corona discharge causes a local streamer discharge. Accordingly, the positive corona discharge has an advantage over the negative corona discharge in evenness of electric discharge. In addition, the negative corona discharge produces several tens of times the amount of ozone produced by the positive corona discharge, thereby increasing environmental load.
FIG. 1A is a schematic view illustrating a related-art corotron corona charger. A charging wire, which serves as a corona discharge electrode, with a diameter of 50 to 100 μm and made of tungsten, is shielded with a metal case forming a gap of about 1 cm there between. A high voltage of 5 to 10 kV is applied to the wire, while an opening is disposed facing a charging target. Thus, positive or negative ions are moved to a surface of the charging target, resulting in charging of the charging target.
FIG. 2A is a graph showing a relation between the charging time and the surface potential of a charging target with respect to the related-art corotron corona charger. It is apparent from FIG. 2A that the corotron corona charger continuously charges the charging target, in other words, continuously discharges electricity. Therefore, the corotron corona charger is not always suitable for charging a charging target to a predetermined potential, whereas it is suitable for constantly charging a charging target. For example, the corotron corona charger is suitable for a transfer charger that continuously charges a recording medium.
FIG. 1B is a schematic view illustrating a related-art scorotron corona charger. The scorotron corona charger was developed for the purpose of reducing unevenness in the resultant potential of a charging target. As illustrated in FIG. 1B, the scorotron charger has a configuration in which a plurality of wires or a mesh is provided as a control electrode in an opening of the metal shield case. The opening is disposed facing a charging target, and a bias voltage is applied to the control electrode.
FIG. 2B is a graph showing a relation between the charging time and the surface potential of a charging target with respect to the related-art scorotron corona charger. It is apparent from FIG. 2B that the surface potential of the charging target is saturated at a predetermined charging time. This is because a voltage applied to the control electrode controls the surface potential of the charging target. The saturation value depends on the voltage applied to the control electrode.
Although having a more complicated configuration and providing a lower charging efficiency than the corotron corona charger, the scorotron corona charger is widely used because of having an advantage in evenness of charging. The control electrode is usually known and described as “charging grid” in electrophotographic image forming apparatuses, and therefore the control electrode may be hereinafter referred to as “charging grid”.
However, the corona charger has the following problems 1) to 3) which are caused by discharge products such as nitrogen oxides (NOx).
1) Environmental Impact
It is known that a negative corona charger typically produces discharge products because substances in the air are reacted upon a negative corona discharge. Specific examples of the discharge products include ozone (O3) and nitrogen oxides (NOx) such as nitrogen monoxide (NO) and nitrogen dioxide (NO2) that are produced by oxidation of nitrogen with ozone. In general, ozone adversely affects the human respiratory system, and humans can generally sense a foul odor of ozone when it is in concentrations of 0.1 ppm or more. Specifically, nitrogen dioxide (NO2) is the worst at adversely affecting the human respiratory system. A permissible level of nitrogen dioxide (NO2) is 0.04 to 0.06 ppm or less per hour on daily average based on environmental standards. Further, nitrogen oxides may be altered into photochemical oxidants (Ox) by a photochemical reaction caused by ultraviolet rays, a permissible level of which is 0.06 ppm or less based on environmental standards. Generally, several tens of ppm of ozone and several ppm of nitrogen oxides are produced in the corona discharge. Consequently, contemporary image forming apparatuses are provided with a filter made of activated carbon or the like so that fewer discharge products are emitted from the image forming apparatus.
2) Impact on Image Quality
2-1) Unevenness in Image Density Immediately Below Corona Charger
During a long-term discharge, discharge products are accumulated on inner walls of a corona charger. When the corona charger is left at rest after the long-term discharge, the discharge products gradually contaminate a charging target, i.e., a photoreceptor, resulting in a difference in surface potential between a surface area of the photoreceptor disposed immediately below the corona charger and the other areas thereof. As a consequence, the resultant image density is uneven. The above-described phenomenon that causes unevenness in the resultant image density prominently occurs in a low-humidity condition of about 20% RH, and rarely occurs at normal temperature and humidity.
Specifically, the surface of the photoreceptor is reversibly reacted with the discharge products, thereby increasing the capacitance or decreasing the resistance of the photoreceptor. As a result, a difference in surface potential is generated. Most photoreceptors cause this phenomenon. In particular, a photoreceptor having a cross-linked surface layer as a protective layer prominently causes the phenomenon.
FIG. 3A is an example of a halftone image of uneven density. FIG. 3B is a graph showing the surface potential of a photoreceptor corresponding to the halftone image illustrated in FIG. 3A. It is apparent from FIGS. 3A and 3B that areas of the halftone image corresponding to the surface areas of the photoreceptor disposed immediately below the corona charger have a higher image density than the other areas.
2-2) Image Blurring
As long as a photoreceptor is charged by electric discharge in an electrophotographic image forming apparatus, image blurring is caused to a greater or lesser extent, resulting in deterioration of resolution of the resultant image, referred to here as image blurring.
Image blurring is caused by adhesion of paper powder to a photoreceptor and by use environment, and is mainly caused by discharge products. Image blurring cannot be completely prevented unless a photoreceptor is charged by a charging method which does not produce ozone and NOx, which is not yet invented. Image blurring can be suppressed to some extent by heating a photoreceptor, however, problems of shortening of the life of the photoreceptor, waste of electric power, upsizing of apparatus and so forth may arise. Alternatively, image blurring can be suppressed to some extent by abrading a surface of a photoreceptor so that discharge products adhered to the surface can be removed. However, contemporary photoreceptors have developed to have a durable surface that is hard to abrade, resulting in insufficient removal of discharge products.
Discharge products adhere to a photoreceptor sparsely at first, but gradually spread thereover. As a consequence, a hygroscopic, low-resistance layer is formed thereon. As described above, image blurring is a phenomenon in which an image, in particular edges thereof, is blurred because an electrostatic latent image is not normally formed. This phenomenon generally occurs when charges diffuse at a surface of the photoreceptor or the periphery thereof. In a case in which a hygroscopic low-resistance layer is formed on or inside the photoreceptor, charges are diffused, resulting in destabilization of the electrostatic latent image. FIG. 4 is a graph showing the surface potential of a photoreceptor when image blurring occurs.
When a surface of a photoreceptor is negatively charged and subsequently exposed to a light beam containing image information, a pair of a hole and an electron is formed in a charge generation layer. The electron then migrates to a conductive substrate, while the hole migrates toward negative charges present on an outermost layer. If the hole meets a low-resistance layer on the way to the outermost layer, the hole may leak laterally without reaching the outermost layer. If the outermost layer itself has a low resistance, the hole may leak laterally on the outermost layer. In these cases, a normal electrostatic latent image cannot be formed because charges, i.e., holes in these cases, are diffused or dissipated.
The lowness of the resistance of the low-resistance layer is preferably as small as possible for the purpose of suppressing diffusion of charges, that is, deterioration of resolution of the resultant image. When the resistance of the low-resistance layer is very low, in particular, when the volume resistivity is 1012 Ω·cm or less, the resolution of the resultant image deteriorates, causing image blurring. Finally, a normal image cannot be produced. At that time, the surface potential of the photoreceptor has a dull pattern, not a square wave pattern, as illustrated in FIG. 4. More specifically, a surface potential of a non-image portion is reduced and that of an image portion is increased, showing a low contrast there between. Such an undesirable phenomenon is further aggravated by time and humidity. In order to produce high resolution images, a photoreceptor is required to have a volume resistivity of 1013 Ω·cm or more and a surface resistivity of 1015 Ω·cm or more.
2-3) Raindrop-Like Marks
An image forming apparatus using the scorotron corona charger sometimes produces an uneven image with raindrop-like marks as illustrated in FIG. 5. Thin lines in a halftone image illustrated in FIG. 5 are raindrop-like marks. This phenomenon occurs when the charge amount is uneven within a small area of a surface of a photoreceptor. Specifically, when high-resistance substances such as toner, silica that is an external additive of the toner, or discharge products are adhered to a charging wire, electric discharge does not occur reliably, causing the above-described phenomenon. When the high-resistance substances described above are adhered to a charging grid, discharged charges cannot flow into the charging grid and flow into the photoreceptor instead, causing the above-described phenomenon. Alternatively, the charging grid itself is charged because the capacitance thereof is increased, thereby excessively charging the photoreceptor locally.
3) Problem in Retaining of Adsorbent
As described above, in order to reduce the emission amount of discharge products from an image forming apparatus, a filter may be provided on an emission path. A charging target disposed immediately below the corona charger is generally contaminated with discharge products, while the corona charger and the charging target are generally disposed facing with each other forming a gap of 1 to 2 mm there between so that the charging target is reliably charged. In order to prevent contamination of the charging target with discharge products, a complicated mechanism is required such that a shield is disposed between the corona charger and the charging target, or the corona charger or the charging target is withdrawn after the electric discharge.
To overcome the disadvantages of such a complicated configuration, one proposed approach involves retaining a zeolite on the charging grid for the purpose of preventing contamination of the charging target with discharge products without such a mechanism. Specifically, the zeolite retained on the charging grid adsorbs discharge products to prevent contamination of a charging target therewith. The charging grid typically retains the zeolite using a binder resin.
However, the zeolite retained on the charging grid may be released therefrom with time, resulting in insufficient removal of discharge products. This is because the discharge products, that is, reactive gases such as ozone and nitrogen oxides may degrade the binder resin with time.
In attempting to solve such problems, Unexamined Japanese Patent Application Publication No. (herein after “JP-A”) 2005-227470 discloses a corona charger, the charging grid of which is made of SUS and coated with a conductive coating composition including an organic binder resin and fine particles of graphite, nickel, and an aluminum-compound. It is disclosed therein that such a configuration prevents corrosion of the charging grid because the conductive coating layer adsorbs discharge products. Accordingly, a charging target is prevented from being contaminated with discharge products. However, since the fine particles in the conductive coating layer adsorb discharge products, the capacity for adsorbing discharge products depends on the number of adsorbing sites in the fine particles, and there is a possibility that the adsorbing sites become buried with long-term use.
Unexamined Japanese Utility Model Application Publication No. 62-089660 discloses a corona charger in which finely partitioned communicating holes are arranged within an opening, and an ozone-adsorbing layer containing an ozone-adsorbing material is further formed on the inner surface of the communication holes. A zeolite and an activated carbon are used as the ozone-adsorbing material. It is disclosed therein that such a configuration prevents diffusion of ozone. However, it is difficult to prevent ozone from diffusing toward a charging target side, possibly contaminating a charging target with ozone.
JP-2003-43894-A discloses an image forming apparatus including a corona charger and a means for removing (adsorbing) discharge products adhered to a charging target, and at least one of a means for preventing adhesion of discharge products to the charging target, a means for preventing lowering of the resistance of the discharge products adhered to the charging target, and a means for reducing the amount of discharge products produced at the periphery of the charging target. Accordingly, multiple members are needed, which is a disadvantage. An embodiment is also disclosed therein in which an adsorbent such as a zeolite is provided between the charging target and the corona charger. However, such an embodiment cannot reliably charge the charging target.
In attempting to effectively reduce the amount of discharge products generated at the periphery of a charger, JP-2001-075338-A discloses an image forming apparatus containing a photocatalyst (i.e., a semiconductor such as titanium oxide) on a surface that faces a discharge wire. The photocatalyst effectively decomposes discharge products so as to reduce the amount thereof. However, the decomposition ability of the photocatalyst may not last for an extended period of time.
JP-2002-278223-A discloses an image forming apparatus including a charging member mainly composed of a catalytic and conductive material such as activated carbon fiber for the purpose of preventing deterioration of image quality under high-humidity conditions, improving durability of a photoreceptor, and preventing generation of discharge products such as ozone and nitrogen oxide. However, an ability of the activated carbon fiber to adsorb discharge products may deteriorate with long-term use. Further, an optional coating layer may not be consistently formed on such an activated carbon fiber because adhesion properties there between may deteriorate with long-term use.
JP-2003-091143-A discloses an image forming apparatus in which a corona charger is disposed below a photoreceptor. An adsorptive and catalytic member is provided on a back side of the corona charger so as to adsorb discharge products. However, the discharge products may also diffuse to a photoreceptor side, which is opposite the back side of the corona charger, resulting in incomplete adsorption of the discharge products.